1. Field of the Invention
The present invention is directed to a bipolar electrode lead for medical applications, particularly for use with an implantable heart pacemaker to deliver electrical stimulation pulses from circuitry within the pacemaker housing to a selected heart location.
2. Description of the Prior Art
The use of unipolar heart pacemaker electrode lead is known, wherein the pacemaker housing serves as the neutral electrode for the stimulation system. This requires that the return current path from the electrode at the lead tip travel through the intervening body tissue between the lead tip and the pacemaker housing. It is known that this can result in unwanted muscle stimulations and/or muscle inhibitions in the tissue surrounding the pacemaker housing.
A known solution to this problem is the use of a bipolar electrode lead, in which the neutral electrode is formed by an exposed ring of electrically conductive material disposed slightly spaced from the active electrode at the tip of the lead. If the tip of the lead is disposed, for example, inside the heart, the complete electrical current path will also be contained inside the heart. In order to keep the current density in the neutral electrode low, the ring must have a relatively large surface area.
The relatively large dimensions of the neutral electrode in such a bipolar arrangement result in considerable stiffening of the lead, which is otherwise extremely flexible. If, for example, the tip of the electrode lead is to be disposed in the left ventricle, the neutral electrode will also be in the left ventricle. Given the large number of bends to which the electrode lead is exposed, such a stiffening represents a high stress, which increases the risk that damage to the insulation, or a rupture of the conductor in the proximity of the stiffening, will occur.
If the bipolar lead is used as an atrial electrode, it can be bent into the shape of a J, so that the tip thereof can be applied in the right atrial appendage. Under such conditions, the neutral electrode cannot be disposed in the region of the bent portion of the lead, because of the aforementioned stiffening problem, and also because the neutral electrode is relatively heavy, compared to the remainder of the lead, so that an unwanted dislocation of the anchoring of the electrode tip can occur. If the neutral electrode is disposed at a greater distance from the distal end (i.e., the tip) of an atrial lead, it will come to lie in the vena cava in most cases. If the neutral electrode is disposed at that location, the current pulses associated with the neutral electrode can influence the phrenic nerve, which contains sensory fibers leading to parts of the pleura and diaphragm. Stimulation of the phrenic nerve can result in hiccups and coughing which are unpleasant for the patient.
A bipolar heart pacemaker electrode lead is described in U.S. Pat. No. 4,295,270 having an electrical conductor consisting of a braided hose which is connected to the neutral electrode. The neutral electrode is formed by a stiff ring, so that this electrode lead has the above-described disadvantages, despite the use of braided hose.
To make a bipolar electrode lead more elastic, a plurality of smaller neutral electrodes can be applied in succession along a portion of the lead. Such a lead is, however, relative complicated to manufacture, and is thus expensive.